st: "vertical filter and sum of data"

Dear Stata Community,
Id like to sum vertically some entries of my data but i dont know how.
Could you please tell me the sintax and structure of how to do this?
Suppose we have a several power transmission lines with several
substations in between, all over the US, indexed from 1 to Ni,
corresponding to a certain line Li. Each substation has a power
consumption (suppose one line per state). Additionaly we have a time
variable MONTH.
The structure of the data is
LINE S_INDEX MONTH POWER
Fixing LINE, MONTH, id like to sum POWER all over (S_INDEX) the line.
Any help will be greatly appreciated. Thank you in advance :) .
Max
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